The invention relates to methods for fabricating field effect devices and more particularly to methods for forming gate electrodes for silicon MOS transistors.
Since the commercial inception of field effect transistors in the early 1970s, the gate dielectric has been silicon dioxide, and more recently, silicon oxynitride. The electrode for these devices has usually been polysilicon. The polysilicon is normally dual-doped and topped with a dopant diffusion barrier of titanium nitride. Recently, it has been proposed that tantalum pentoxide be substituted for silicon dioxide as a dielectric. See C. Hu, Elec. Dev. Letters, September 1998, p. 341-42, incorporated herein as if set forth in its entirety. However, tantalum pentoxide leakage current increases upon elevated temperature processing as it loses oxygen. Annealing in an oxidizing gas such as O2 or N2O normally reverses this degradation. Titanium nitride is an ineffective barrier to this oxygen loss, especially above about 600C, as titanium nitride starts to decompose at about that temperature. Thus there is a need for an improved gate electrode material in high-density silicon MOS transistor technology when tantalum pentoxide is used as the gate dielectric.
We have developed an improved MOS gate structure for silicon MOS transistor IC devices with tantalum pentoxide or stacked tantalum pentoxide dielectrics. The structure includes a two level composite of WSi and WSiN and a three level composite of WSi/WSiN/WSi. The tungsten silicide layer provides electrical conductivity for the gate structure, and the tungsten silicon nitride layer is an effective barrier to oxygen diffusion, especially at elevated temperatures to about 800C. This gate structure suffers none of the elevated temperature problems of titanium nitride. A useful feature of the improved structure is that it can be manufactured in a single deposition tool with convenient in-situ processing of all layers of the composite.